Systems and methods for adaptive switching frequency control in switching-mode power conversion systems
Switching-mode power conversion system and method thereof. The system includes a primary winding configured to receive an input voltage and a secondary winding coupled to the primary winding. Additionally, the system includes a compensation component configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage, and a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal. Moreover, the system includes a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal, and a first switch configured to receive the drive signal and affect a first current flowing through the primary winding.
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This application claims priority to U.S. Provisional No. 61/084,538, filed Jul. 29, 2008, commonly assigned, incorporated by reference herein for all purposes.
2. BACKGROUND OF THE INVENTIONThe present invention is directed to switching-mode power conversion systems. More particularly, the invention provides systems and methods for adaptive switching frequency control. Merely by way of example, the invention has been applied to off-line switching-mode flyback power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
To meet certain international power conservation requirements, power supplies need to have very low levels of standby power consumption. For example, the averaged power efficiency should be high at 25%, 50%, 75%, and 100% load conditions. In switching-mode power conversion systems, power loss mainly results from the switching loss of the power switches, the conduction loss of the power switches, the core loss of the transformer and inductors, and the power loss of the snubbers. All of these types of power losses are proportional to the switching frequency. Additionally, the core loss of the transformer and inductors and the power loss of the snubbers also depend on the material used.
As shown in
Hence, the switching frequency fSW depends on IC and ID.
Referring to
Specifically, the switching loss PSW often is given by
where fsw is the switching frequency, and VIN is an input voltage for the primary winding. As shown in
According to Equation 4, for a given power switch such as MOSFET, the switching loss increases with the input voltage as a quadratic function. Hence, lowering the switching frequency can reduce the switching loss, especially for high input voltages.
In another example, the conduction loss Pcond is
where IPK is the peak current that the switch conducts at the end of the switch-on period, and IPK(0) is the current that the switch conducts at the beginning of the switch-on period. Additionally, R is the on-resistance of the switch, ton is the conduction period, and fsw is the switching frequency. The product of ton·fSW is called duty cycle. For a given output power and a given switching frequency, a lower input voltage can result in larger duty cycle thus higher conduction loss.
Moreover, for the flyback power conversion system 100, the power P delivered to the output is, for example,
where L is the inductance of the primary winding, and VIN is an input voltage for the primary winding. For example, the input voltage VIN is the rectified line voltage. For a given output power P, increasing the switching frequency fsw can result in decreasing magnitude for (IPK2−IPK2(0)), thus lower conduction loss according to Equation 5.
Therefore, it is highly desirable to improve techniques related to conversion efficiency of a power conversion system.
3. BRIEF SUMMARY OF THE INVENTIONThe present invention is directed to switching-mode power conversion systems. More particularly, the invention provides systems and methods for adaptive switching frequency control. Merely by way of example, the invention has been applied to off-line switching-mode flyback power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
According to one embodiment of the present invention, a switching-mode power conversion system includes a primary winding configured to receive an input voltage and a secondary winding coupled to the primary winding. Additionally, the system includes a compensation component configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage, and a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal. Moreover, the system includes a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal, and a first switch configured to receive the drive signal and affect a first current flowing through the primary winding. The drive signal is associated with a switching frequency, and the switching frequency varies with the input voltage in magnitude.
According to another embodiment of the present invention, a switching-mode power conversion system includes a primary winding configured to receive an input voltage, and a secondary winding coupled to the primary winding. Additionally, the system includes a compensation component including a voltage sensing component, a current generator and an oscillator, and the oscillator is coupled to the current generator. The compensation component is configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage. Moreover, the system includes a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal, and a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal. Also, the system includes a first switch configured to receive the drive signal and affect a first current flowing through the primary winding. The voltage sensing component is configured to receive the input voltage and generate a detected voltage based on at least information associated with the input voltage, and the current generator is configured to receive the detected voltage and generate a second current and a third current based on at least information associated with the detected voltage. Each of the second current and the third current varies with the input voltage in magnitude. Also, the control signal is associated with a switching frequency, and the switching frequency decreases with the increasing input voltage in magnitude.
According to yet another embodiment of the present invention, a method for regulating a switching frequency for a switching-mode power conversion system includes receiving an input voltage by a primary winding and by a compensation component. The compensation component includes a voltage sensing component, a current generator and an oscillator, the oscillator coupled to the current generator. Additionally, the method includes generating at least a clock signal based on at least information associated with the input voltage, receiving at least the clock signal by a signal generator, generating at least a control signal based on at least information associated with the clock signal, and receiving at least the control signal by a gate driver. Moreover, the method includes generating a drive signal based on at least information associated with the control signal, receiving the drive signal by a first switch, and affecting a first current flowing through the primary winding. The process for generating at least a clock signal includes receiving the input voltage by the voltage sensing component, generating a detected voltage based on at least information associated with the input voltage, receiving the detected voltage by the current generator, and generating a second current and a third current based on at least information associated with the detected voltage. Also, each of the second current and the third current varies with the input voltage in magnitude. The control signal is associated with a switching frequency, and the switching frequency decreases with the increasing input voltage in magnitude.
Many benefits are achieved by way of the present invention over conventional techniques. Certain embodiments of the present invention provide adaptive line-voltage compensated switching frequency control. Some embodiments of the present invention use the line voltage to modulate the switching frequency. For example, a high line voltage results in low switching frequency, and a low line voltage results in high switching frequency. Certain embodiments of the present invention provide switching frequency control methods to reduce power loss in switching-mode power conversion systems including but not limited to off-line power supplies. Some embodiments of the present invention can improve conversion efficiency for both low and high line voltages.
Depending upon embodiment, one or more of these benefits may be achieved. These benefits and various additional objects, features and advantages of the present invention can be fully appreciated with reference to the detailed description and accompanying drawings that follow.
The present invention is directed to switching-mode power conversion systems. More particularly, the invention provides systems and methods for adaptive switching frequency control. Merely by way of example, the invention has been applied to off-line switching-mode flyback power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
The input-voltage sensing component 270 receives an input voltage 276, represented by Vin. For example, the input voltage 276 is the rectified line voltage. In another example, the input voltage 276 is received by the primary winding 260. In response, the input-voltage sensing component 270 generates a detected voltage 278 according to an embodiment. For example, the detected voltage 278 is received by the input-voltage compensated current generator 280. In another example, the current generator 280 processes information associated with the detected voltage 278 and outputs one or more reference currents 282 based on at least information associated with the detected voltage 278.
As shown in
The clock signal 292 and the ramp signal 294 are fed into the PWM generator 220. The PWM generator 220 also receives an amplified signal 212 from the error amplifier 210 and a current-sensing signal 222 from the terminal 202 (i.e., the terminal CS). As shown in
In one embodiment, the PWM generator 220 processes information associated with the signals 292, 294, 212 and 222, and generates a PWM signal 224, which is received by the logic control component 230. For example, the PWM signal 224 has a switching frequency equal to the oscillation frequency of the clock signal 292 and the ramp signal 294.
In another example, the logic control component 230 outputs a control signal 232 to the gate driver 240. The gate drier 240 sends a drive signal 242 to the switch 250 according to an embodiment. In response, according to another embodiment, the switch 250 is turned on or off in order to control a current 264 that flows through the primary winding 260.
Referring back to
Returning to
As shown in
In one embodiment, the input-voltage-compensated controller 770 receives the detected voltage 278, processes information associated with the detected voltage 278, and outputs control signals 772 and 774 to the current sources 710 and 715 respectively. For example, the current source 710 generates a charge current IC based on at least information associated with the control signal 772. In another example, the current source 715 generates a discharge current ID based on at least information associated with the control signal 774.
As shown in
In one embodiment, the NAND gates 740 and 745 form a RS flip-flop for latching switching states. In another embodiment, the comparators 730 and 735 perform voltage clamping functions. For example, the comparator 730 receives a reference voltage VH and the ramping signal 294. In yet another example, the comparator 735 receives a reference voltage VL and the ramping signal 294.
In one embodiment, the input-voltage sensing component 270 includes the resistors 272 and 274. In another embodiment, the input-voltage sensing component 270 receives the input voltage 276, and in response generates the detected voltage 278. The detected voltage 278 is, for example, received by the comparator 870 and the transistor 810. The comparator 870 also receives a reference voltage 872 represented by VREF.
As shown in
where I0 represents the magnitude of the current 812, and R1 and R2 represent resistance of the resistors 274 and 272, respectively. Thus, a current 822 that flows from the transistor 820 to the transistor 810 is
where I1 represents the current 812. As shown in
where α is a coefficient that is determined by the current mirror ratio.
Additionally, the current 822 is mirrored to generate a current 832 by a current mirror that includes the transistors 820 and 830. For example, the current 822 is represented by I2. In another example, the current 832 flows from the transistor 830 to the transistor 850. Furthermore, the current 832 is mirrored to generate the discharge current ID by a current mirror that includes the transistors 850 and 860. For example, the discharge current ID is
where β is a coefficient that is determined by the ratios of the two current mirrors, one of which includes the transistors 820 and 830, and the other of which includes the transistors 850 and 860. As shown in
Referring to
The input-voltage sensing component 970, through the resistor 974, receives an input voltage 976, represented by Vin. For example, the input voltage 976 is the rectified line voltage. In another example, the input voltage 976 is received by the primary winding 960. In response, the input-voltage sensing component 970 generates a detected signal 978 according to an embodiment. For example, the detected signal 978 is a detected voltage. In another example, the detected signal 978 is a detected current. In yet another example, the detected signal 978 is received by the input-voltage compensated current generator 980. In yet another example, the current generator 980 processes information associated with the detected signal 978 and outputs one or more reference currents 982 based on at least information associated with the detected signal 978. In addition, the constant current generator 985 is biased to the ground voltage through the resistor 972, and generates one or more constant currents 988 according to an embodiment.
As shown in
The clock signal 992 and the ramp signal 994 are fed into the PWM generator 920. The PWM generator 920 also receives an amplified signal 912 from the error amplifier 910 and a current-sensing signal 922 from the terminal 902 (i.e., the terminal CS). As shown in
In one embodiment, the PWM generator 920 processes information associated with the signals 992, 994, 912 and 922, and generates a PWM signal 924, which is received by the logic control component 930. For example, the PWM signal 924 has a switching frequency equal to the oscillation frequency of the clock signal 992 and the ramp signal 994.
In another example, the logic control component 930 outputs a control signal 932 to the gate driver 940. The gate drier 940 sends a drive signal 942 to the switch 950 according to an embodiment. In response, according to another embodiment, the switch 950 is turned on or off in order to control a current 964 that flows through the primary winding 960. As shown in
In one embodiment, the input-voltage-compensated controller 1070 receives the detected signal 978, processes information associated with the detected signal 978, and outputs control signals 1072 and 1074 to the current sources 1010 and 1020 respectively. For example, the current source 1010 generates a current IC1 based on at least information associated with the control signal 1072. In another example, the current source 1020 generates a current ID1 based on at least information associated with the control signal 1074. Additionally, the current sources 1015 and 1025 generate currents IC and ID respectively, according to an embodiment.
As shown in
In one embodiment, the NAND gates 1040 and 1045 form a RS flip-flop for latching switching states. In another embodiment, the comparators 1030 and 1035 perform voltage clamping functions. For example, the comparator 1030 receives a reference voltage VH and the ramping signal 994. In yet another example, the comparator 1035 receives a reference voltage VL and the ramping signal 994.
According to another embodiment of the present invention, a switching-mode power conversion system includes a primary winding configured to receive an input voltage and a secondary winding coupled to the primary winding. Additionally, the system includes a compensation component configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage, and a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal. Moreover, the system includes a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal, and a first switch configured to receive the drive signal and affect a first current flowing through the primary winding. The drive signal is associated with a switching frequency, and the switching frequency varies with the input voltage in magnitude. For example, the system is implemented according to at least
In yet another example, the switching frequency decreases with the increasing input voltage in magnitude. In yet another example, the input voltage is a rectified line voltage. In yet another example, the compensation component includes at least a voltage sensing component and a current generator. The voltage sensing component is configured to receive the input voltage and generate a detected voltage based on at least information associated with the input voltage, and the current generator is configured to receive the detected voltage and generate at least a second current based on at least information associated with the detected voltage corresponding to a detected voltage magnitude, the second current corresponding to a first current magnitude. The first current magnitude varies with the detected voltage magnitude.
In yet another example, the voltage sensing component includes a first resistor and a second resistor. The first resistor is associated with a first terminal and a second terminal, and the second resistor associated with a third terminal and a fourth terminal. The first terminal is biased to the input voltage, and the second terminal and the third terminal are coupled together at a node associated with the detected voltage. In yet another example, the current generator is further configured to generate a third current based on at least information associated with the detected voltage. The third current corresponds to a second current magnitude, and the second current magnitude varies with the detected voltage magnitude. In yet another example, the second current decreases with the increasing input voltage in magnitude, and the third current decreases with the increasing input voltage in magnitude. In yet another example, the compensation component further includes an oscillator configured to receive at least the second current and generate at least the clock signal based on at least information associated with the second current. The clock signal corresponds to an oscillation frequency, and the oscillation frequency varies with the input voltage in magnitude. In yet another example, the switching frequency is equal to the oscillation frequency in magnitude. In yet another example, the compensation component is further configured to generate a ramping signal, and the signal generator is further configured to receive the ramping signal corresponding to the oscillation frequency.
According to yet another embodiment of the present invention, a switching-mode power conversion system includes a primary winding configured to receive an input voltage, and a secondary winding coupled to the primary winding. Additionally, the system includes a compensation component including a voltage sensing component, a current generator and an oscillator, and the oscillator is coupled to the current generator. The compensation component is configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage. Moreover, the system includes a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal, and a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal. Also, the system includes a first switch configured to receive the drive signal and affect a first current flowing through the primary winding. The voltage sensing component is configured to receive the input voltage and generate a detected voltage based on at least information associated with the input voltage, and the current generator is configured to receive the detected voltage and generate a second current and a third current based on at least information associated with the detected voltage. Each of the second current and the third current varies with the input voltage in magnitude. Also, the control signal is associated with a switching frequency, and the switching frequency decreases with the increasing input voltage in magnitude. For example, the system is implemented according to at least
In another example, the oscillator includes at least a capacitor, a second switch, and a third switch. The second current is a charge current for the capacitor if the second switch is closed and the third switch is open, and the third current is a discharge current for the capacitor if the second switch is open and the third switch is closed. In yet another example, the voltage sensing component includes a first resistor and a second resistor. The first resistor is associated with a first terminal and a second terminal, and the second resistor is associated with a third terminal and a fourth terminal. The first terminal is biased to the input voltage, and the second terminal and the third terminal are coupled together at a node associated with the detected voltage. In yet another example, the second current decreases with the increasing input voltage in magnitude, and the third current decreases with the increasing input voltage in magnitude. In yet another example, the oscillator is configured to receive the second current and the third current and generate the clock signal and a ramping signal based on at least information associated with the second current and the third current. Both the clock signal and the ramping signal correspond to an oscillation frequency, and the oscillation frequency varies with the input voltage in magnitude.
According to yet another embodiment of the present invention, a method for regulating a switching frequency for a switching-mode power conversion system includes receiving an input voltage by a primary winding and by a compensation component. The compensation component includes a voltage sensing component, a current generator and an oscillator, the oscillator coupled to the current generator. Additionally, the method includes generating at least a clock signal based on at least information associated with the input voltage, receiving at least the clock signal by a signal generator, generating at least a control signal based on at least information associated with the clock signal, and receiving at least the control signal by a gate driver. Moreover, the method includes generating a drive signal based on at least information associated with the control signal, receiving the drive signal by a first switch, and affecting a first current flowing through the primary winding. The process for generating at least a clock signal includes receiving the input voltage by the voltage sensing component, generating a detected voltage based on at least information associated with the input voltage, receiving the detected voltage by the current generator, and generating a second current and a third current based on at least information associated with the detected voltage. Also, each of the second current and the third current varies with the input voltage in magnitude. The control signal is associated with a switching frequency, and the switching frequency decreases with the increasing input voltage in magnitude. For example, the method is implemented according to at least
In another example, the process for generating at least a clock signal further includes receiving the second current and the third current by the oscillator, and generating at least the clock signal based on at least information associated with the second current and the third current. The clock signal corresponds to an oscillation frequency, and the oscillation frequency varies with the input voltage in magnitude. In yet another example, the process for generating at least the clock signal based on at least information associated with the second current and the third current includes charging a capacitor by the second current if a second switch is closed and a third switch is open, and discharging the capacitor by the third current if the second switch is open and the third switch is closed. The oscillator includes at least the capacitor, the second switch, and the third switch. In yet another example, the second current decreases with the increasing input voltage in magnitude, and the third current decreases with the increasing input voltage in magnitude. In yet another example, the detected voltage is proportional with the input voltage in magnitude.
Many benefits are achieved by way of the present invention over conventional techniques. Certain embodiments of the present invention provide adaptive line-voltage compensated switching frequency control. Some embodiments of the present invention use the line voltage to modulate the switching frequency. For example, a high line voltage results in low switching frequency, and a low line voltage results in high switching frequency. Certain embodiments of the present invention provide switching frequency control methods to reduce power loss in switching-mode power conversion systems including but not limited to off-line power supplies. Some embodiments of the present invention can improve conversion efficiency for both low and high line voltages.
Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.
Claims
1. A switching-mode power conversion system, the system comprising:
- a primary winding configured to receive an input voltage;
- a secondary winding coupled to the primary winding;
- a compensation component configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage;
- a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal;
- a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal; and
- a first switch configured to receive the drive signal and affect a first current flowing through the primary winding;
- wherein: the drive signal is associated with a switching frequency; the switching frequency varies with the input voltage in magnitude; and the compensation component includes at least a voltage sensing component and a current generator; wherein: the voltage sensing component is configured to receive the input voltage and generate a detected voltage based on at least information associated with the input voltage; the current generator is configured to receive the detected voltage and generate at least a second current based on at least information associated with the detected voltage corresponding to a detected voltage magnitude, the second current corresponding to a first current magnitude; and the first current magnitude varies with the detected voltage magnitude; wherein: the current generator is further configured to generate a third current based on at least information associated with the detected voltage, the third current corresponding to a second current magnitude; and the second current magnitude varies with the detected voltage magnitude.
2. The system of claim 1 wherein the switching frequency decreases with the increasing input voltage in magnitude.
3. The system of claim 1 wherein the input voltage is a rectified line voltage.
4. The system of claim 1 wherein:
- the voltage sensing component includes a first resistor and a second resistor, the first resistor associated with a first terminal and a second terminal, the second resistor associated with a third terminal and a fourth terminal;
- the first terminal is biased to the input voltage; and
- the second terminal and the third terminal are coupled together at a node associated with the detected voltage.
5. The system of claim 1 wherein:
- the second current decreases with the increasing input voltage in magnitude; and
- the third current decreases with the increasing input voltage in magnitude.
6. The system of claim 1 wherein:
- the compensation component further includes an oscillator configured to receive at least the second current and generate at least the clock signal based on at least information associated with the second current, the clock signal corresponding to an oscillation frequency; and
- the oscillation frequency varies with the input voltage in magnitude.
7. The system of claim 6 wherein the switching frequency is equal to the oscillation frequency in magnitude.
8. The system of claim 6 wherein:
- the compensation component is further configured to generate a ramping signal; and
- the signal generator is further configured to receive the ramping signal corresponding to the oscillation frequency.
9. A switching-mode power conversion system, the system comprising:
- a primary winding configured to receive an input voltage;
- a secondary winding coupled to the primary winding;
- a compensation component including a voltage sensing component, a current generator and an oscillator, the oscillator coupled to the current generator, the compensation component being configured to receive the input voltage and generate at least a clock signal based on at least information associated with the input voltage;
- a signal generator configured to receive at least the clock signal and generate at least a control signal based on at least information associated with the clock signal;
- a gate driver configured to receive at least the control signal and generate a drive signal based on at least information associated with the control signal; and
- a first switch configured to receive the drive signal and affect a first current flowing through the primary winding;
- wherein: the voltage sensing component is configured to receive the input voltage and generate a detected voltage based on at least information associated with the input voltage; the current generator is configured to receive the detected voltage and generate a second current and a third current based on at least information associated with the detected voltage; each of the second current and the third current varies with the input voltage in magnitude; the control signal is associated with a switching frequency; and the switching frequency decreases with the increasing input voltage in magnitude.
10. The system of claim 9 wherein:
- the oscillator includes at least a capacitor, a second switch, and a third switch;
- the second current is a charge current for the capacitor if the second switch is closed and the third switch is open; and
- the third current is a discharge current for the capacitor if the second switch is open and the third switch is closed.
11. The system of claim 9 wherein:
- the voltage sensing component includes a first resistor and a second resistor, the first resistor associated with a first terminal and a second terminal, the second resistor associated with a third terminal and a fourth terminal;
- the first terminal is biased to the input voltage; and
- the second terminal and the third terminal are coupled together at a node associated with the detected voltage.
12. The system of claim 9 wherein:
- the second current decreases with the increasing input voltage in magnitude; and
- the third current decreases with the increasing input voltage in magnitude.
13. The system of claim 9 wherein:
- the oscillator is configured to receive the second current and the third current and generate the clock signal and a ramping signal based on at least information associated with the second current and the third current, both the clock signal and the ramping signal corresponding to an oscillation frequency; and
- the oscillation frequency varies with the input voltage in magnitude.
14. A method for regulating a switching frequency for a switching-mode power conversion system, the method comprising:
- receiving an input voltage by a primary winding and by a compensation component including a voltage sensing component, a current generator and an oscillator, the oscillator coupled to the current generator;
- generating at least a clock signal based on at least information associated with the input voltage;
- receiving at least the clock signal by a signal generator;
- generating at least a control signal based on at least information associated with the clock signal;
- receiving at least the control signal by a gate driver;
- generating a drive signal based on at least information associated with the control signal;
- receiving the drive signal by a first switch; and
- affecting a first current flowing through the primary winding;
- wherein the process for generating at least a clock signal includes: receiving the input voltage by the voltage sensing component; generating a detected voltage based on at least information associated with the input voltage; receiving the detected voltage by the current generator; and generating a second current and a third current based on at least information associated with the detected voltage;
- wherein: each of the second current and the third current varies with the input voltage in magnitude; the control signal is associated with a switching frequency; and the switching frequency decreases with the increasing input voltage in magnitude.
15. The method of claim 14 wherein the process for generating at least a clock signal further includes:
- receiving the second current and the third current by the oscillator; and
- generating at least the clock signal based on at least information associated with the second current and the third current;
- wherein: the clock signal corresponds to an oscillation frequency; and the oscillation frequency varies with the input voltage in magnitude.
16. The method of claim 15 wherein the process for generating at least the clock signal based on at least information associated with the second current and the third current includes:
- charging a capacitor by the second current if a second switch is closed and a third switch is open; and
- discharging the capacitor by the third current if the second switch is open and the third switch is closed;
- wherein the oscillator includes at least the capacitor, the second switch, and the third switch.
17. The method of claim 14 wherein:
- the second current decreases with the increasing input voltage in magnitude; and
- the third current decreases with the increasing input voltage in magnitude.
18. The method of claim 14 wherein the detected voltage is proportional with the input voltage in magnitude.
Type: Grant
Filed: Jul 27, 2009
Date of Patent: Jul 24, 2012
Patent Publication Number: 20100027299
Assignee: On-Bright Electronic (Shanghai) Co., Ltd. (Shanghai)
Inventor: Lieyi Fang (Shanghai)
Primary Examiner: Jue Zhang
Attorney: Jones Day
Application Number: 12/510,021
International Classification: H02M 3/335 (20060101);